Field
The disclosed technology relates to an integrated circuit and a method of manufacturing the integrated circuit. In particular, the integrated circuit includes photo-sensitive areas for acquiring an image and filters for selectively transmitting different wavelengths of electro-magnetic radiation to different photo-sensitive areas.
Description of the Related Technology
Many applications require acquisition of spectral images of an object. Spectral imaging implies that the spectral content of an image of the object is extracted and incident electro-magnetic radiation relating to multiple narrow ranges of wavelengths is detected. The spectral imaging may for instance be used in analysis of imaged objects, such as for determination whether a substance having a specific spectral profile is present in the object.
Multispectral imaging refers to spectral imaging using a plurality of discrete wavelength bands, whereas hyperspectral imaging refers to imaging narrow spectral wavelength bands over a continuous spectral range. Hyperspectral imaging may also often use a larger number of spectral bands than what is used in multispectral imaging.
It is commonly desired that spectral imaging is performed for each part of an object. Thus, image data may need to be gathered in three dimensions, two spatial dimensions combined with a spectral dimension. This implies that large amounts of image data need to be gathered and sets requirements on an imaging device for gathering the image data.
US PG Publication No. 2011/0181757 discloses an apparatus for multispectral imaging. The apparatus includes a monochromatic two-dimensional image sensor and an optical filter wheel arranged in front of the image sensor. The filter wheel consists of multiple filter segments and may be moved so different filter segments are arranged in a light path. Thus, different frames captured by the image sensor may hold different spectral information. However, such an apparatus is bulky and requires movable parts for obtaining image data in three dimensions.
US PG Publication No. 2011/0049340 discloses a substrate having detector cells that detect radiation incident thereon and a filter module providing multiple filters that are each aligned with a detector cell. The filter module is made up of two mirrors that are spaced apart by a spacer membrane and the filter module further has a plurality of interference filters, the thickness of the spacer membrane being constant for any filter and varying from one filter to another. Radiation is reflected multiple times in the mirrors at each side of the spacer membrane, and multiple rays at an output of the filter cause interference depending on a phase shift introduced in the spacer membrane. The many interfering rays lead to a selective optical filter. Thus, the device allows for gathering spectral image data for a large number of different wavelength bands by the thickness of the spacer membrane being different for different detector cells. However, the mirror material may have a relatively high absorption coefficient, which implies that a mirror layer needs to be thin. Simultaneously, the mirror layer needs to be sufficiently thick in order to achieve the desired reflectivity.
US PG Publication No. 2008/0042782 discloses an integrated narrow bandpass filter array. The filter array includes a substrate, a lower mirror stack, a spacer array, and an upper mirror stack. The spacer array is an array of varied thickness and is used to control a pass band for each narrow bandpass filter and realizes the integration of narrow bandpass filters with different pass bands on a single substrate. The mirror stacks are formed by alternating layers of high and low refractive index, respectively.
It is difficult to design a multitude of combined filters that provide a possibility of gathering image data holding spectral information for a broad spectrum of wavelengths. In particular, the filters may absorb large proportions of the electro-magnetic radiation, which implies that spectral imaging may only work in conditions having large intensities of electro-magnetic radiation. Also, or alternatively, the mirrors at each side of a spacer may only partially reflect electro-magnetic radiation in portions of a broad spectrum of wavelengths. This implies that electro-magnetic radiation within such portions of the spectrum of wavelengths may instead be transmitted towards the detector such that the filter is not satisfactorily providing narrow bandpass filters over the entire range of the broad spectrum.